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Type conversion of epitaxial GaAs layers after heavy ion MeV implantation and annealing
Nuclear Instruments North-Holland
and Methods
in Physics Research
BS9/60
1103
(1991) 110331105
Type conversion of epitaxial GaAs layers after heavy ion MeV implantation and annealing F.G.
Moore,
P.B. Klein
Nuval Research Laboratov,
and
Washington.
H.B.
Dietrich
DC 203 7S- 5000, USA
and annealing of thick epitaxial layers of n-type GaAs has been found to result in Heavy ion (erbium, lh7Er) MeV implantation the conversion of a surface layer within the erbium profile to p-type. Similar effects in originally p-type and undoped, high resistivity layers are not observed. In addition, no active role is ascribed to the erbium in providing mobile holes. The source of the type conversion is attributed to the transfer of Si,, donors to Si,, acceptor sites as the result of the implantation and annealing.
High energy implantation of dopants in GaAs has received a substantial amount of study [l] and there is some basic understanding of the damage and annealing mechanisms associated with MeV implantation of light ions [2,3] in GaAs. While cascades in the tracks of heavy ions (at keV energies) are known to be efficient at creating lattice damage [4], there have been no systematic studies of these phenomena at higher energies. Prominent among the heavier elements, and of recent interest as dopants in the III-Vs are the lanthanide rare earths [5-71. In the present study the rare earth erbium (Er) was implanted into a variety of epitaxial GaAs samples. In silicon doped n-type layers the implantation and annealing invariably caused a conversion to p-type conductivity over a portion of the implant profile. We believe that this behavior can be explained by the implantation and annealing induced transfer of Sio, to Si AI- In contrast to these observations in n-type materials, no significant changes in the as-grown electrical properties of p-type and undoped layers was documented. Two multiple energy implant schedules were used. The first (“high” fluence) was done at 1.5 and 3 MeV with fluences of 3.74 and 6.6 x 10” cm-‘, respectively, to generate a = 1 pm thick region with an Er concentration of = 2 X 10” cme3. The second (“low” fluence) waskdone at 1.5, 3 and 5 MeV with fluences of 2. 1.3 and 1.3 x lOI cm-‘, respectively, and resulted in a = 1.5 pm thick profile of Er at a concentration of = 5 x 10” cm-‘. Implantation was followed by a furnace anneal to remove implantation damage and (potentially) to activate the Er as a dopant. Before annealing all samples were capped with 1000 A of Si,N,. The annealing atmosphere was forming gas (10% I-I,. 90% N,). After annealing, electrochemical C-V 016~-5~3X/91/~03.50
6 1991 - Elsevier Science Publishers
profiling (Polaron) measurements were made and the results compared to the original, as-grown profiles. Photoluminescence (PL) experiments were carried out at 10 K with 647 nm excitation from a Kr-ion laser. The extinction length of the excitation light is = 0.3 pm. The PL emission was dispersed with a 0.75 m grating monochromator and detected with either a GaAs photomultiplier or a cooled Ge pin diode. Fig. 1 gives the carrier concentration profile for a high fluence Er implant into an OMCVD (organometallic chemical vapor deposition) layer grown doped with silicon to n = 3 X 10’s cm-‘. Superimposed is the as-grown carrier profile and the erbium implantation
as-orown. n = 3x10” c117.a
\I
0.4
n-type
1.2
0.8 Depth
1.6
(irm)
Fig. 1. Polaron profile measuring carrier concentration and SIMS measuring Er concentrations. The as-grown carrier concentration before done into epitatial
B.V. (North-Holland)
implantation is also shown. Implants were GaAs doped with silicon to a concentration of n=3x10’8cm-3. VIII. SEMICONDUCTORS
profile as measured by secondary ion mass spectrometry (SIMS). Note that the first = 0.5 km of the sample now exhibits p-type conduction. A corresponding behavior is seen for a similar Er implant into material doped to 11= 1 X lO”cmas shown in fig. 2. Surprisingly. the p-type carrier concentration is approximately the same as that of the highly Si doped material indicated in fig. I. The low fluence implants into identical. as-grown material also exhibit p-type behavior with the converted p-type region pinned at = 5 x 10lh cm ‘. Shown in fig. 3 are photoluminescence (PL) spectra obtained on the sample shown in fig. 2. Both the virgin material and material after anneal at 840” C exhibit free-to-bound and donor-acceptor pair emissions associated with the C,, acceptor. as determined by their spectral positions [S]. After implantation and annealing the spectrum is dominated by a transition to a shallow acceptor identified from its spectral position as Si.,. This behavior indicates that the implantation and annealing process has caused the amphoteric silicon impurity to move from the Ga site (donor) to the As site (acceptor). The self-compensation which results may be largely responsible for the type conversion noted in all of the (as-grown) n-type samples that we have examined. The figure also shows a shift in the peak of the somewhat broadened bound exciton spectrum from a position indicative of donor bound excitons (before implantation and annealing) to a position associated with acceptor bound excitons afterwards. This behavior is consistent with the observed conversion from n- to p-type, due to a transfer of silicon donors to acceptor sites. Polaron profiles of Er implanted epitaxial material originally beryllium doped from p = 1 X 10lh cm-” to
P-type
0.4
0.6 Depth
1.2
16
(vN
Fig. 2. Polaron profile measuring carrier concentration and SIMS measuring Er concentrations. Implants were done into epitaxial GaAs doped with silicon to a concentration of n = 8 x IO” cm ‘. also shown is the Polaron profile for the as-grown material.
OM435 WGaAS =lxlO”cm’SI
Anneal only, 840”
1.4
1.44
1 A8
Energy
1.52
(eV)
Fig. 3. Photoluminescence spectrum (10 K) of the n-type (n = 8 X 10” cm-‘) material shown In fig. 2. The bpectrum of the as-grown material is dominated by PL from C,,, acceptors. while the Er-implanted material is dominated hy Si,, acceptors. The accepted positions of the donor-acceptor. free-tobound and bound excitons are indicated.
p = 5 X 10” cm-“.
were also obtained. No significant changes in the p-type carrier concentration profiles of these samples was observed after implantation and annealing. This is entirely consistent with the conclusion that the observed type conversion is related to the transfer of the Si donors to As sites. Certainly Er itself is not effective at providing mobile carriers: With an atomic concentration of 3 X 10’s cm ‘. some significant increase in the carrier concentration of the p-type layers would have been observed. Supporting this idea are the results of measurements on several undoped. high-resistivity epitaxial layers. These materials were implanted with Er and individual samples annealed at different temperatures from 650&900°C. After annealing. all of these samples remained high resistive. While there has been a report of Er acting as a p-type dopant in InP [9]. our results do not support this view. although this aforementioned behavior was reported for a very different set of samples that were grown by liquid phase epitaxy from an Er rich melt. More recent work indicates that Er may in fact act as an effective electron trap [lo]. The amphoteric doping behavior of silicon in GaAs has been well documented [11.12]. For GaAs doped with silicon via ion implantation this phenomenon normally occurs in samples having an implant profile with a high (= 5 X 10” cm-j or greater) atomic concentration and which. as a result of the high level of ion-induced damage. have been subjected to a high temperature anneal. The self-compensation that results from this amphoteric behavior normally manifests itself by generating a level of n-type doping substantially lower than would be expected if all of the implanted silicon
were acting as substitutional donors [13]. However, there documenting anomalous p-type activations in the near-surface regions of MeV implanted silicon profiles in semi-insulating GaAs 1141. These anomalous activations were ascribed to an incomplete removal of implantation damage and were not seen when annealing was done at sufficiently high temperatures. This work involved the implantation of silicon into semi-insulating material and differs from the present work. where the implantation of a heavy ion (Er) into n-type (as-grown) GaAs : Si was seen to result in a type conversion. There has also been a report of type conversion in bulk. silicon doped GaAs implanted with oxygen and chlorine [15]. These results, too, were ascribed to an incomplete removal of damage during annealing. If these two previous observations of type conversion were attributed to the presence of Si,, acceptors (which is not inconsistent the remainder of these studies) then these observations along with our own would have in common the occupation of Si,, sites under conditions of high damage. either generated by high fluence, MeV light ion implants or by moderate Sluence MeV implants of a heavy ion (Et). This would then suggest a damage related mechanism in GaAs favoring incorporation of silicon onto arsenic sites and thus resulting in the silicon dopant being highly self compensated. A somewhat puzzling point is that the phenomenon seems to be stable to annealing temperatures of at least 800°C in the case of MeV Er implants but unstable with MeV silicon. oxygen or chlorine implants annealed to slightly higher temperatures. The damage caused by implantation is typically modelled using a heterogeneous mechanism for heavy ions (e.g. Er) and a homogeneous mechanism for light ions (e.g. Si) [16]. Given this. it is not unreasonable. to expect that the stability (during annealing) of the type conversion process to be related to the specific detail of the damage present in the crystal after implantation which in turn is related to (among other parameters) the mass of the incident ion. It should he noted at this point that we have assumed from the appearance of Si,, in the PL data that the p-type conversion occurs totally through self compensation. Although this may be correct, it is quite possible that some of the silicon donors are compensated by defects that remain after the annealing stage or even by erbium which is well known to getter donor impurities. In addition, the p-type conductivity is undoubtedly due to both Si,, and C,,. as both appear in the PL spectrum after implantation and annealing, although the Si,,, is clearly dominant as shown in fig. 3. in conclusion, we have presented the first evidence linking the amphoteric nature of silicon in GaAs with the type conversion of n-type GaAs after implantation is a report
and annealing. Although the particular method by which the process occurs is not presently understood, it is postulated that the phenomenon is driven by the high concentration of damage resulting from the MeV implantation of heavy ions (n”Er).
Acknowledgements This work was accomplished while the first author held a National Research Council/Naval Research Laboratory Postdoctoral Associateship. Work at the Naval Research Laboratory was sponsored in part by the Office of Naval Research. The authors would like to thank Dave Simons for his assistance with SIMS measurements.
References
VI P.E. Thompson.
these Proceedings (7th Int. Conf. on Ion Beam Modification of Materials, Knoxville, TN. USA. 1990) Nucl. Instr. and Meth. B59/60 (1991) 592. PI T. Lee. G. Braunstein and S. Chen, Mater. Res. Sot. Symp. Proc. 126 (1988) 183. 131 C.R. Wie. T.A. Tombrello and T. Vreeland Jr.. Phys. Rev. B33 (1986) 4083. and [41 M. Poate and J.S. Williams, in: Ion Implantation Beam Processing (Academic Press. New York. 19X4) p. 13. [51 J.P. Galtier. M.N.B. Charasse. B. Groussin, T. Benyattou and G. Guillot, Appl. Phys. Lett. 55 (1989) 2105. (61 A. Rolland. A. Le Corre. P.N. Favennec. M. Gauneau. B. Lambert, D. Lecrosnier, H. L’haridon. D. Moutonnet and C. Rochaid. Electron. Lett. 24 (1988) 956. and K. Takahei. [71 P.S. Whitney, K. Uwai. H. Nakagome Electron. Lett. 24 (198X) 740. ISI 5. Ashen. P.J. Dean, D.T.J. Hurle, J.B. Mullin, A.M White and P.D. Greene. J. Phys. Chem. Solids 36 (1975) 1041. and K.W. Benz. J. [91 W. Korber, J. Weber. A. HangIeiter Crystal Growth 79 (1986) 741. A. Le Corre, Y. Toudic, C. L’homer. G. PO1 B. Lambert. Grandpierre and M. Gunneau, J. Phys. Cond. Matt. 2 ( 1990) 479. P.P. Pronko and SC. Ling. Appt. Phys. [111 R.S. Battacharya, Lrtt. 42 (1983) 8X0. A. K. Rai, Y.K. Yeo. P.P. Pronko. SC. wd R.S. Battacharya. Ling. S.R. Wilson and Y.S. Park, J. Appl. Phys. 54 (1983) 2329. C.P. Wu, C.W. Magee, S.Y. iI31 Ci. Liu, E.C. Douglas, Narayan. ST. Jolly, F. Kolondra and S. Jain, RCA Rev. 41 (1980) 227. H.B. Dietrich. M. Spencer and D.C. [I41 P.E. Thompson. Ingram, SPIE Proc. 530 (1985) 35. iI51 T.T. Bardin. J.G. Pronko. F.A. Junga, W.G. Gpyd. A.J. Mardinly. F. Xiong and T.A. Tombrello. Nucl. Instr. and Meth. B24/25 (19X7) 54X. [I61 K. Sadana, Nucl. Instr. and Meth. 87/X (1985) 375.
VIII. SEMICONDUCTORS
and Methods
in Physics Research
BS9/60
1103
(1991) 110331105
Type conversion of epitaxial GaAs layers after heavy ion MeV implantation and annealing F.G.
Moore,
P.B. Klein
Nuval Research Laboratov,
and
Washington.
H.B.
Dietrich
DC 203 7S- 5000, USA
and annealing of thick epitaxial layers of n-type GaAs has been found to result in Heavy ion (erbium, lh7Er) MeV implantation the conversion of a surface layer within the erbium profile to p-type. Similar effects in originally p-type and undoped, high resistivity layers are not observed. In addition, no active role is ascribed to the erbium in providing mobile holes. The source of the type conversion is attributed to the transfer of Si,, donors to Si,, acceptor sites as the result of the implantation and annealing.
High energy implantation of dopants in GaAs has received a substantial amount of study [l] and there is some basic understanding of the damage and annealing mechanisms associated with MeV implantation of light ions [2,3] in GaAs. While cascades in the tracks of heavy ions (at keV energies) are known to be efficient at creating lattice damage [4], there have been no systematic studies of these phenomena at higher energies. Prominent among the heavier elements, and of recent interest as dopants in the III-Vs are the lanthanide rare earths [5-71. In the present study the rare earth erbium (Er) was implanted into a variety of epitaxial GaAs samples. In silicon doped n-type layers the implantation and annealing invariably caused a conversion to p-type conductivity over a portion of the implant profile. We believe that this behavior can be explained by the implantation and annealing induced transfer of Sio, to Si AI- In contrast to these observations in n-type materials, no significant changes in the as-grown electrical properties of p-type and undoped layers was documented. Two multiple energy implant schedules were used. The first (“high” fluence) was done at 1.5 and 3 MeV with fluences of 3.74 and 6.6 x 10” cm-‘, respectively, to generate a = 1 pm thick region with an Er concentration of = 2 X 10” cme3. The second (“low” fluence) waskdone at 1.5, 3 and 5 MeV with fluences of 2. 1.3 and 1.3 x lOI cm-‘, respectively, and resulted in a = 1.5 pm thick profile of Er at a concentration of = 5 x 10” cm-‘. Implantation was followed by a furnace anneal to remove implantation damage and (potentially) to activate the Er as a dopant. Before annealing all samples were capped with 1000 A of Si,N,. The annealing atmosphere was forming gas (10% I-I,. 90% N,). After annealing, electrochemical C-V 016~-5~3X/91/~03.50
6 1991 - Elsevier Science Publishers
profiling (Polaron) measurements were made and the results compared to the original, as-grown profiles. Photoluminescence (PL) experiments were carried out at 10 K with 647 nm excitation from a Kr-ion laser. The extinction length of the excitation light is = 0.3 pm. The PL emission was dispersed with a 0.75 m grating monochromator and detected with either a GaAs photomultiplier or a cooled Ge pin diode. Fig. 1 gives the carrier concentration profile for a high fluence Er implant into an OMCVD (organometallic chemical vapor deposition) layer grown doped with silicon to n = 3 X 10’s cm-‘. Superimposed is the as-grown carrier profile and the erbium implantation
as-orown. n = 3x10” c117.a
\I
0.4
n-type
1.2
0.8 Depth
1.6
(irm)
Fig. 1. Polaron profile measuring carrier concentration and SIMS measuring Er concentrations. The as-grown carrier concentration before done into epitatial
B.V. (North-Holland)
implantation is also shown. Implants were GaAs doped with silicon to a concentration of n=3x10’8cm-3. VIII. SEMICONDUCTORS
profile as measured by secondary ion mass spectrometry (SIMS). Note that the first = 0.5 km of the sample now exhibits p-type conduction. A corresponding behavior is seen for a similar Er implant into material doped to 11= 1 X lO”cmas shown in fig. 2. Surprisingly. the p-type carrier concentration is approximately the same as that of the highly Si doped material indicated in fig. I. The low fluence implants into identical. as-grown material also exhibit p-type behavior with the converted p-type region pinned at = 5 x 10lh cm ‘. Shown in fig. 3 are photoluminescence (PL) spectra obtained on the sample shown in fig. 2. Both the virgin material and material after anneal at 840” C exhibit free-to-bound and donor-acceptor pair emissions associated with the C,, acceptor. as determined by their spectral positions [S]. After implantation and annealing the spectrum is dominated by a transition to a shallow acceptor identified from its spectral position as Si.,. This behavior indicates that the implantation and annealing process has caused the amphoteric silicon impurity to move from the Ga site (donor) to the As site (acceptor). The self-compensation which results may be largely responsible for the type conversion noted in all of the (as-grown) n-type samples that we have examined. The figure also shows a shift in the peak of the somewhat broadened bound exciton spectrum from a position indicative of donor bound excitons (before implantation and annealing) to a position associated with acceptor bound excitons afterwards. This behavior is consistent with the observed conversion from n- to p-type, due to a transfer of silicon donors to acceptor sites. Polaron profiles of Er implanted epitaxial material originally beryllium doped from p = 1 X 10lh cm-” to
P-type
0.4
0.6 Depth
1.2
16
(vN
Fig. 2. Polaron profile measuring carrier concentration and SIMS measuring Er concentrations. Implants were done into epitaxial GaAs doped with silicon to a concentration of n = 8 x IO” cm ‘. also shown is the Polaron profile for the as-grown material.
OM435 WGaAS =lxlO”cm’SI
Anneal only, 840”
1.4
1.44
1 A8
Energy
1.52
(eV)
Fig. 3. Photoluminescence spectrum (10 K) of the n-type (n = 8 X 10” cm-‘) material shown In fig. 2. The bpectrum of the as-grown material is dominated by PL from C,,, acceptors. while the Er-implanted material is dominated hy Si,, acceptors. The accepted positions of the donor-acceptor. free-tobound and bound excitons are indicated.
p = 5 X 10” cm-“.
were also obtained. No significant changes in the p-type carrier concentration profiles of these samples was observed after implantation and annealing. This is entirely consistent with the conclusion that the observed type conversion is related to the transfer of the Si donors to As sites. Certainly Er itself is not effective at providing mobile carriers: With an atomic concentration of 3 X 10’s cm ‘. some significant increase in the carrier concentration of the p-type layers would have been observed. Supporting this idea are the results of measurements on several undoped. high-resistivity epitaxial layers. These materials were implanted with Er and individual samples annealed at different temperatures from 650&900°C. After annealing. all of these samples remained high resistive. While there has been a report of Er acting as a p-type dopant in InP [9]. our results do not support this view. although this aforementioned behavior was reported for a very different set of samples that were grown by liquid phase epitaxy from an Er rich melt. More recent work indicates that Er may in fact act as an effective electron trap [lo]. The amphoteric doping behavior of silicon in GaAs has been well documented [11.12]. For GaAs doped with silicon via ion implantation this phenomenon normally occurs in samples having an implant profile with a high (= 5 X 10” cm-j or greater) atomic concentration and which. as a result of the high level of ion-induced damage. have been subjected to a high temperature anneal. The self-compensation that results from this amphoteric behavior normally manifests itself by generating a level of n-type doping substantially lower than would be expected if all of the implanted silicon
were acting as substitutional donors [13]. However, there documenting anomalous p-type activations in the near-surface regions of MeV implanted silicon profiles in semi-insulating GaAs 1141. These anomalous activations were ascribed to an incomplete removal of implantation damage and were not seen when annealing was done at sufficiently high temperatures. This work involved the implantation of silicon into semi-insulating material and differs from the present work. where the implantation of a heavy ion (Er) into n-type (as-grown) GaAs : Si was seen to result in a type conversion. There has also been a report of type conversion in bulk. silicon doped GaAs implanted with oxygen and chlorine [15]. These results, too, were ascribed to an incomplete removal of damage during annealing. If these two previous observations of type conversion were attributed to the presence of Si,, acceptors (which is not inconsistent the remainder of these studies) then these observations along with our own would have in common the occupation of Si,, sites under conditions of high damage. either generated by high fluence, MeV light ion implants or by moderate Sluence MeV implants of a heavy ion (Et). This would then suggest a damage related mechanism in GaAs favoring incorporation of silicon onto arsenic sites and thus resulting in the silicon dopant being highly self compensated. A somewhat puzzling point is that the phenomenon seems to be stable to annealing temperatures of at least 800°C in the case of MeV Er implants but unstable with MeV silicon. oxygen or chlorine implants annealed to slightly higher temperatures. The damage caused by implantation is typically modelled using a heterogeneous mechanism for heavy ions (e.g. Er) and a homogeneous mechanism for light ions (e.g. Si) [16]. Given this. it is not unreasonable. to expect that the stability (during annealing) of the type conversion process to be related to the specific detail of the damage present in the crystal after implantation which in turn is related to (among other parameters) the mass of the incident ion. It should he noted at this point that we have assumed from the appearance of Si,, in the PL data that the p-type conversion occurs totally through self compensation. Although this may be correct, it is quite possible that some of the silicon donors are compensated by defects that remain after the annealing stage or even by erbium which is well known to getter donor impurities. In addition, the p-type conductivity is undoubtedly due to both Si,, and C,,. as both appear in the PL spectrum after implantation and annealing, although the Si,,, is clearly dominant as shown in fig. 3. in conclusion, we have presented the first evidence linking the amphoteric nature of silicon in GaAs with the type conversion of n-type GaAs after implantation is a report
and annealing. Although the particular method by which the process occurs is not presently understood, it is postulated that the phenomenon is driven by the high concentration of damage resulting from the MeV implantation of heavy ions (n”Er).
Acknowledgements This work was accomplished while the first author held a National Research Council/Naval Research Laboratory Postdoctoral Associateship. Work at the Naval Research Laboratory was sponsored in part by the Office of Naval Research. The authors would like to thank Dave Simons for his assistance with SIMS measurements.
References
VI P.E. Thompson.
these Proceedings (7th Int. Conf. on Ion Beam Modification of Materials, Knoxville, TN. USA. 1990) Nucl. Instr. and Meth. B59/60 (1991) 592. PI T. Lee. G. Braunstein and S. Chen, Mater. Res. Sot. Symp. Proc. 126 (1988) 183. 131 C.R. Wie. T.A. Tombrello and T. Vreeland Jr.. Phys. Rev. B33 (1986) 4083. and [41 M. Poate and J.S. Williams, in: Ion Implantation Beam Processing (Academic Press. New York. 19X4) p. 13. [51 J.P. Galtier. M.N.B. Charasse. B. Groussin, T. Benyattou and G. Guillot, Appl. Phys. Lett. 55 (1989) 2105. (61 A. Rolland. A. Le Corre. P.N. Favennec. M. Gauneau. B. Lambert, D. Lecrosnier, H. L’haridon. D. Moutonnet and C. Rochaid. Electron. Lett. 24 (1988) 956. and K. Takahei. [71 P.S. Whitney, K. Uwai. H. Nakagome Electron. Lett. 24 (198X) 740. ISI 5. Ashen. P.J. Dean, D.T.J. Hurle, J.B. Mullin, A.M White and P.D. Greene. J. Phys. Chem. Solids 36 (1975) 1041. and K.W. Benz. J. [91 W. Korber, J. Weber. A. HangIeiter Crystal Growth 79 (1986) 741. A. Le Corre, Y. Toudic, C. L’homer. G. PO1 B. Lambert. Grandpierre and M. Gunneau, J. Phys. Cond. Matt. 2 ( 1990) 479. P.P. Pronko and SC. Ling. Appt. Phys. [111 R.S. Battacharya, Lrtt. 42 (1983) 8X0. A. K. Rai, Y.K. Yeo. P.P. Pronko. SC. wd R.S. Battacharya. Ling. S.R. Wilson and Y.S. Park, J. Appl. Phys. 54 (1983) 2329. C.P. Wu, C.W. Magee, S.Y. iI31 Ci. Liu, E.C. Douglas, Narayan. ST. Jolly, F. Kolondra and S. Jain, RCA Rev. 41 (1980) 227. H.B. Dietrich. M. Spencer and D.C. [I41 P.E. Thompson. Ingram, SPIE Proc. 530 (1985) 35. iI51 T.T. Bardin. J.G. Pronko. F.A. Junga, W.G. Gpyd. A.J. Mardinly. F. Xiong and T.A. Tombrello. Nucl. Instr. and Meth. B24/25 (19X7) 54X. [I61 K. Sadana, Nucl. Instr. and Meth. 87/X (1985) 375.
VIII. SEMICONDUCTORS